This invention relates to methods and apparatus for producing nitrogen fertilizer solutions.
It is known that ammonium and nitrate ions are the major nitrogen forms absorbed by plant roots. It has been described in Biological Review, 1978. 55:465-510, that each ion may produce different yield and physiological responses within plant and cropping systems. Moreover, certain common crops and categories of different crops respond to different ratios of the two ions in the soil solution by providing increased plant growth and yield when the proper ion ratio exists in the soil solution during the crop growth period. For example, corn grows most rapidly with 1:3 ammonium-nitrate ion ratio in a normal temperature soil solution while tomatoes grow most efficiently with a large preponderance of nitrate nitrogen in the soil solution.
It is thus known to be beneficial to apply nitrate fertilizers to crops and to exercise control of the ammonium:nitrate ion ratio in the soil solution of a particular crop in accordance with that crop's needs.
The nitrogen fertilizer industry, in most countries, uses anhydrous ammonia as a principal component. This is because anhydrous ammonia: (1) has relatively low initial cost; (2) is the raw material for other synthetic nitrogen fertilizers; (3) has high nitrogen content, the content being approximately eighty-two percent (82%) by weight; and (4) has physical characteristics which make long distance pipeline, rail and truck transport efficient.
Anhydrous ammonia is a liquefied gas and because of that it has the several disadvantages under some circumstances, such as: (1) requiring special storage and handling facilities which are so expensive as to limit storage of anhydrous ammonia at the farm level and the local retail level; (2) requiring specialized subsurface application equipment to avoid ammonia loss at application time; (3) not being suitable for application in irrigation waters because it results in calcium carbonate deposits in the irrigation equipment and prohibitive amounts of fertilizer nitrogen are lost to the atmosphere through volatilization; (4) the time during a cropping season when it can be applied is limited because of the subsurface application requirements; and (5) because it must be applied at certain times, logistical problems and economic burdens are created in the industry.
One of the economic problems occurs because many soils, and especially coarser irrigated soils, cannot hold enough nitrogen applied as a single preplant ammonia application to sustain economic crop yields. Thus, it is necessary to apply split applications of nitrogen fertilizer. The time during the crop cycle when ammonia can be side dressed as a split application is limited by the growth of the crop plants because the operation requires knifing the ammonia gas into the soil.
It is known to use non-pressure nitrogen solutions to reduce some of these difficulties because they adapt readily to simple handling, storage and application methods. Moreover, the application of non-pressure fertilizer nitrogen solutions can be accomplished in concert with crop needs and this elicits a better crop growth response for each unit of applied nitrogen than the normal method of applying nitrogen in a concentrated ammonia band in the soil at the beginning of or shortly after the start of a crop cycle.
Non-pressure nitrogen solutions can be applied to the crop as a top dressing mechanically or in water used to irrigate the crop so it can be applied in a practical manner as the crop needs nitrogen.
However, non-pressure nitrogen solution fertilizers manufactured and applied by prior art techniques have a disadvantage in that a delivered unit of nitrogen as non-pressure nitrogen solution costs nearly double that for nitrogen as anhydrous ammonia. There have been many prior art attempts to reduce the cost of non-pressure nitrogen solutions.
In the prior art, one commercial system for providing non-pressure nitrogen fertilizer solutions for farm use consists of producing a relatively concentrated ammonium nitrate solution and blending it with a urea solution for shipment to dealers with subsequent distribution to and use at the farm level. The normal solution sold to the farmer contains from 28% to 32% nitrogen by weight, which is a low concentration product for long distance transportation. It must also be handled through a multi-tier distribution system.
The ammonium nitrate solution is produced by oxidizing ammonia with air, in the presence of a catalyst, to nitrogen oxide, oxidizing the nitrogen oxide to nitrogen dioxide and absorbing the nitrogen dioxide in water to produce nitric acid. In some prior art embodiments, this process is carried out at high pressure to make the desired acid concentration and reduce the size of the nitrogen oxide conversion and absorption system. Some systems include 30 to 35 stages of oxide conversion and absorption. In some systems, the concentrated nitric acid is reacted with ammonia to form an ammonium nitrate solution.
These commercial processes have the disadvantages of: (1) being expensive; (2) having considerable air pollution hazard; and (3) resulting in an expensive product. The cost of the product is increased for several reasons, such as: (1) the cost of making concentrated nitric acid and concentrated urea solutions is high; (2) there is cost in controlling atmospheric pollution; (3) there are substantial raw material losses in the manufacturing process; and (4) transporting and distributing a low concentration 28% to 32% nitrogen product over long distances to market is expensive.
In another prior art process, nitrogen is separated from the exhaust of tractors or other engines on a farm and the nitrogen is used as top dressing or for combination with other materials to form nitrogenous fertilizers. These processes are described in U.S. Pat. Nos. 2,943,419, 2,947,112 and 3,099,898.
These processes have the disadvantages of: (1) being limited in the amount of fertilizer obtainable; (2) being obtainable at only certain times; (3) producing undesirable nitrite nitrogen in the end product; (4) being limited in capacity to produce specific fertilizer nitrogen forms for cropping situations where that is desired; (5) being limited in the amount of fertilizer that can be produced economically by the amount of exhaust available from tractors or other engines while they are used for other purposes and being obtainable only after such use unless fuel is wasted by using the tractors or engines only to produce exhaust; and (6) requiring expensive and complicated apparatus to effect their reduction to practice.
Still another prior art process uses a multi-purpose irrigation-hydroelectric project as: (1) a base for manufacturing nitrogen solution fertilizer; (2) a vehicle for delivering said fertilizer to farms; (3) a means for correcting soil and water alkalinity problems of irrigated farms with nitrogen fertilizer; and (4) a means of fertilizing irrigated crops with nitrogen fertilizer.
The hydroelectric power not used for normal community use is used to prepare ammonia in a conventional manner. The ammonia is then partially oxidized, the remainder of the gas stream is mixed with the oxides formed in the partial oxidation and the mixture is absorbed in water to make a nitrogen solution fertilizer. Alternatively, electricity is used to make nitrogen oxides by the electric arc process and the oxides of nitrogen treated to make a nitrogen solution fertilizer. This process is described in U.S. Pat. Nos. 2,028,172 and 2,088,869.
This process has the disadvantages of: (1) not being suitable for use on individual farms or local sites serving only a few farms and thus being limited in application; (2) causing some damage in use; and (3) being inefficient in several respects.
Firstly, the process of U.S. Pat. Nos. 2,028,172 and 2,088,869 is not suitable for use on individual farms or local sites serving only a few farms and thus is limited in application for several reasons such as: (1) it is applicable only to large-scale, multi-purpose irrigation-hydroelectric projects; (2) it requires large-scale, multi-purpose dam sites for water and power and thus consideration of too large a number of individual farm's separate needs; (3) it does not permit practical tailoring of the ions produced to specific crop needs unless the same crop is used over a wide area which causes difficulties with diseases and insects; and (4) it requires the handling and conversion of nitrogen oxides at a central irrigation-hydroelectric dam site.
Secondly, the process of U.S. Pat. Nos. 2,028,178 and 2,088,869 causes damage in use because: (1) it causes extensive groundwater pollution; (2) it requires the transportation in open ditches of corrosive and environmentally dangerous materials over long distances; and (3) it produces nitrites in the end product in concentrations that are harmful when applied to crops.
Thirdly, the process of U.S. Pat. Nos. 2,028,178 and 2,088,869 is inefficient because: (1) it has large fertilizer losses in transit; (2) it provides a low yield of usable nitrate ions; (3) it converts oxides to nitrates in (a) conventional trickle towers which are large if operated at atmospheric pressure or expensive if operated at conventional high pressure; (b) turbine infall or outfall which results in prohibitive nitrogeneous raw material losses in the form of nitrogen oxides gas loss to the atmosphere; or (c) irrigation ditches which results in high levels of nitrite ions and the high loss of nitrogenous gasses to the atmosphere; and (4) it has a low yield of usable nitrogen fertilizer compounds from the initial manufacturing process because the means taught for rationing of ammonia to nitric nitrogen in consonance with the relative alkalinity or acidity of water or soil is by controlling the proportion of ammonia which is oxidized.
In the last case, the effluent gases from the oxidizing catalyst comprise the desired mixture of ammonia-nitric-oxide-oxygen-inert gases which gas mixture is then cooled and passed into a trickle tower or otherwise brought into contact with the water or a portion of the water to be utilized for irrigation.
This process has the disadvantage of resulting in a large proportion of the ammonia and nitric oxide produced by the process being converted to inert nitrogen gas and water, thus making the process yield so low as to be impractical. The approach was apparently selected because it would interface conveniently with the inherent characteristics of a hydroelectric-irrigation project.
Because the decomposition of ammonium nitrite is an ionic reaction in which the ammonium ion combines with the nitrite ion to form gaseous nitrogen as a product, scrubbers or sources of turbulence designed to remove nitrogen oxides from gas streams or to form ammonium nitrate and ammonium nitrite using ammoniated liquids suffer substantial raw material loses by decompositions of ammonium nitrite in this manner, especially at higher oxide concentrations because they generally operated at too low pH's.
The multi-purpose irrigation-hydroelectric projects are necessarily large-scale (U.S. Bureau of Reclamation, Reclamation Project Data; historical, technical and statistical information on reclamation projects; U.S. Government Printing Office 1948) with each project serving as a minimum approximately 20,000 acres and generally substantially more than 50,000 acres of farmland. They are restricted as to general usefulness because they must use large-scale, multi-purpose dam sites for water and power, thereby requiring the consideration of a large number of individual farmer's separate needs in the fertilizer management thus making the project unmanageable.
Such projects are useful only for large-scale applications and therefore do not permit practical tailoring of the ions produced to specific crop needs unless the same crop is used by many farmers over a wide area in the irrigation district. However, this results in agronomic difficulties with diseases and insects.
Such projects have the disadvantages of: (1) requiring the handling and conversion of nitrogen oxides at a central irrigation-hydroelectric dam site as contrasted to processing in a small locality or on a farm where the control is necessary to agronomic and economic success; (2) causing extensive ground water pollution by requiring the transport of nitrogen fertilizer to farms in district irrigation canals; (3) requiring the transportation of corrosive and environmentally dangerous materials over long distances to farms in open ditches; (4) causing losses of nitrogen fertilizer in delivery to the farm that are economically prohibitive; and (5) having a yield of usable nitrate ions that is low.
This system produces nitrites in the end product in concentrations that would be harmful when applied to crops unless conventional trickle towers, which to be efficient are large in size if operated at atmospheric pressure and require expensive construction and maintenance if operated at conventional high pressure, are used for oxidation to nitrate. Trickle towers normally have 30 to 35 stages for operation at atmospheric pressure. The use of turbine infall or outfall for oxidation conversion results in prohibitive nitrogeneous raw material losses in the form of nitrogen oxide gas loss to the atmosphere.
Still other methods have been developed or proposed for manufacturing and providing non-pressure nitrogen fertilizer solutions for a farm, which purport to improve the current commercial system for converting nitrogen oxides to usable nitrate ions.
One such method, described in U.S. Pat. No. 2,102,136, proposes passing nitrogen oxides into a flow of acidified inorganic nitrate, preferably a nitrate of ammonia solution, to which is continually added ammonia. The fluid is recycled through a tower until it is increased in bulk to the desired point and then periodically drawn off. After neutralization, it is sent to storage as an ammonium nitrate solution. The solution produced is then used in the normal commercial manner. This process requires either a large number of stages or high pressure operation so it does not provide economic or logistical advantages over the existing commercial system.
Another prior art process disclosed in U.S. Pat. No. 1,541,808 teaches the direct formation of an ammonium nitrate solution for subsequent utilization in the normal commercial manner. It proposes the use of an ammonium bicarbonate or carbonate solution as an absorbing medium for nitrogen oxides, for the purpose of forming an ammonium nitrate-nitrite solution which solution is treated with nitric acid and air in an ammonium nitrite oxidation tower to convert the nitrite to nitrate.
A main disadvantage of this process is the loss of nitrogen oxide raw material during the absorption of the last 30% of the nitrogen oxides. The patent recommends that for some purposes it is advisable to make some other use of these oxides. The process also has the disadvantages of requiring: (1) the use, and thus making, of carbonate or bicarbonate solution as an absorbing liquid; and (2) requiring the use of substitute oxide recovery methods at low nitrogen oxide feed gas concentrations.
In the early designs for atmospheric-pressure nitric acid plants, alkali absorption of various amounts of the oxides of nitrogen was carried out to reduce the size of the plants. It was common to absorb the last 20 to 30% of the oxides from a nitric acid process in a base other than ammoniated solutions. Ammonia was not used because of the loss of gases as nitrogen by decomposition of ammonium nitrite, which is an ionic process forming nitrogen and water. The process of U.S. Pat. No. 1,541,808 has the same problems.
Still other patents describe processes for making ammonium nitrate which rely upon the removal of nitrogen oxides from the effluent gases of industrial process plants and particularly nitric acid plants for the main purpose of preventing nitrogen oxide pollution of the atmosphere, while at the same time deriving a useful nitrogenous end product from the nitrogen oxides removal process. These processes teach: (1) the use of absorbing solutions of acidic and ammonical ammonium nitrate, which solutions are employed in various manners; and (2) the use of nitric acid as an oxidant for converting ammonium nitrite formed in the processes to useful ammonium nitrate. Examples of those processes are described in U.S. Pat. Nos. 3,453,071; 4,151,265 and 4,219,534.
U.S. Pat. No. 3,453,071 has the disadvantages of: (1) requiring that the molar ratio of NO to NO.sub.2 be approximately 1:1 prior to passing the feed gases into the ammoniacal ammonium nitrate absorbing solution in order for the process to work; and (2) requiring multiple ammoniated absorption stages. During the normal operation of a nitric acid plant, the ratio of NO to NO.sub.2 in the tail gas varies considerably with ambient operating condition variations, it is very difficult to control the oxide ratios at 1:1 under practical operating conditions and the ratios would likely vary considerably between the suggested multiple stages of the suggested process thus complicating the practice of the process commercially. Moreover, this process has low nitrogen oxide recovery and marginal performance in the removal of nitrogen oxides from the gas stream.
The process of U.S. Pat. Nos. 4,151,265 and 4,129,534 applies to the removal of nitrogen oxide gases from the tail gas of a nitric acid plant which gas typically contains from 2,000 to 6,000 ppm of nitrogen oxides. In the process, incoming tail gas is reacted first with acidic ammonium nitrate, the exit gas from the first stage is reacted with ammonical ammonium nitrate liquid and exit gas of the second stage is contacted in a third stage with acidified water.
Under low pressure or low oxygen conditions, a nitrogen dioxide rich gas is added to the first stage to cause the exit gas of the first stage to have a nitrogen dioxide to nitric oxide ratio of about 2:1 thereby facilitating the effectiveness of the second stage. The reactant liquids of the system are circulated and proportioned in a manner which minimizes the formation of ammonium nitrite and oxidizes the ammonium nitrite which is formed to ammonium nitrate in an acidic ammonium nitrate liquid.
This process is effective for recovering nitrogen oxides from typical high pressure or medium pressure nitric acid plant tail gases while forming ammonium nitrate solution as an end product. The process is proposed for nitric acid plants operating at from three to seven atmospheres pressure and with typical nitrogen oxides content of the tail gases of about 2,000 to 6,000 rpm.
This process for making ammonium nitrate solution has the disadvantages of: (1) being applicable only to typical tail gas streams; (2) requiring extensive circulation and control of reaction liquids; (3) depending on high pressure operation conditions; (4) requiring the use of nitrogen oxide free gas to the first stage, which is rich in nitrogen dioxide content; (5) not conviently producing a speicified ratio of nitrate to ammonium ions in the end product; and (6) producing a low concentration nitrogen solution end product.
Firstly, this process is applicable only to typical tail gas streams which are low in content of nitrogen oxides. Thus it is limited in the amount of ammonium nitrate that can be produced while handling a given volume of gases. Because of this, it is limited in production capacity and economic return on plant investment unless used primarily for pollution control.
Secondly, it requires extensive circulation and control of reaction liquids. To provide this circulation and control, sophisticated controls and pumps must be used thus causing excessive cost.
Thirdly, it requires high pressure operation conditions in the preferred embodiment for conversion of nitrogen oxides to usable nitrate ions. This requirement causes the absorber size and construction to be similar to that for a normal commercial nitric acid process.
Fourthly, it requires the use of nitrogen oxide feed gas to the first stage, which is rich in nitrogen dioxide content, when operating under low pressure or low oxygen conditions. This complicates the use of the process for low pressure operating conditions.
Fifthly, it does not conveniently produce a specified ratio of nitrate to ammonium ions in the end product at a local or farm use site. Thus, it is not useful as an economic and convenient source of nitrogen solution fertilizer containing the desired ratio of nitrogen ions at the locality or farm for a particular cropping use.
Sixthly, it produces a low concentration nitrogen soltion end product which must be transported at high cost over long distances to a farm market if it is to be used as nitrogen solution fertilizer.